Mapping of tidal current and associated nonlinear currents in the Xiangshan Bay by coastal acoustic tomography

Herein, tidal current and associated nonlinear currents were measured using nine coastal acoustic tomography (CAT) systems surrounding the tomography domain in Xiangshan Bay from 12:00 on December 10 to 12:00 on December 11, 2019 (local time, UTC+8). The distributions of M2 and M4 tidal currents, which were dominant in the bay, were reconstructed by the coast-fitting method (CFM) using the differential travel time data obtained along the 36 transmission paths. The tidal currents for both M2 and M4 showed a significant contrast in current directions and phases between the eastern and western halves of the tomography domain. For M2, progressive- and standing-type waves were generated in the eastern and western halves, respectively. The M4 currents were out-of-phase (with a time lag of 3 h) between the eastern and western halves. Furthermore, it was found that the M4 current in the western half was considerably influenced by the 6-h seiches resonating in the Xiangshan Fjord on the western side of the tomography domain. The observation and inversion errors were significantly smaller than the amplitudes of the M2 and M4 currents.